The present invention relates to a method for producing a filler-loaded thermoplastic resin composite free from the problem of segregation of a resin component and a filler component during the mixing operation.
As is needless to say, thermoplastic resins are molded into shaped articles by the principle of plastic deformation of the plasticized or molten resin with an external shearing force. The process of molding of a thermoplastic resin is carried out very efficiently by a conventional method such as injection molding, extrusion molding, blow molding, calendering, inflation and the like according to the types of the resin and the desired shapes of the products. When the article is to be shaped with the resin alone with optional additive ingredients in rather small amounts, the resin material in particulate form and the additives may be introduced directly into the molding machine, e.g. extruder machine or injection machine, where the resin is melted by heating and extruded or injected through a die or into a metal mold.
On the other hand, many of the plastic articles are shaped with a resin composite loaded with considerably large amounts of a filler. When a thermoplastic resin and a filler are introduced into a molding machine as a mere blend of both in particulate or powdery forms, there unavoidably takes place segregation of the components leading to an uneven distribution of the components owing to the large difference in the powder characteristics of both components so that no uniformity in the quality of the shaped products can be ensured in such a single-step process. Accordingly, it is a common practice in the production of shaped articles of thermoplastic resins loaded with a filler than the resin and the filler are first blended and kneaded together at a temperature higher than the softening or melting temperature of the resin into a uniform mass by use of a batch-wise mixing machine such as a Banbury mixer or a continuous mixing machine such as a Farrel continuous mixer or double-screw mixer and then shaped into pellets and shaping of articles in molding machines is carried out with such pellets.
The above described conventional process is composed of the pelletizing step and the shaping step and is effective from the standpoint of obtaining uniformity in the quality of shaped articles in relation to the distribution of the filler in the thermoplastic resin. A problem in this two-step process is the increased costs for the fabrication of the shaped articles in comparison with a single-step process as a natural consequence of the lengthy process as well as due to the increased loss of the materials in the process. What is worse, the thermoplastic resin must twice undergo melting once in the pelletizing step and again in the shaping step resulting in greater thermal degradation of the resin polymer than in a single-step production in addition to the excessive consumption of heat energy. This problem of thermal degradation is more severe when the thermoplastic resin is liable to thermal degradation such as ABS resins. Another problem in the filler-loaded resin pellets is that, depending on the nature of the filler, the pellets absorb moisture more or less during storage or transportation after pelletizing when the filler is hygroscopic of affinitive to moisture. Moisture absorption of pellets is of course undesirable due to the possible foaming of the resin composition in the step of shaping in which the resin is heated and melted. In extreme cases, shaping of films by the inflation method or shaping of articles by blow molding is almost impossible because of the rupture of the resin layer with the foams of water vapor. The moisture absorption of the filler-loaded thermoplastic resin pellets is largely responsible to the so-called silver streaking appearing in articles shaped by injection molding. Therefore, filler-loaded resin pellets must be stored under moisture-proof sealing or must be dried by heating directly before introduction into molding machines to avoid problems.
The above disadvantages can be somewhat mitigated by a recently proposed method in which pellets of the resin with filler loading much higher than in the final formulation are prepared in advance and these highly filler-loaded pellets are blended with calculated amounts of the resin per se just before introduction into a molding machine to give a desired final filler content. This method is, however, also a two-step process and hence far from satisfactory.
Several attempts have been made to carry out a single-step production process in which a mere blend of a particulate or powdery thermoplastic resin and a filler is directly introduced into a molding machine without being pelletized. Such an attempt is of course successful when a molding machine of specific design is used such as a double-screw extruder or kneader-extruder in which kneading and extrusion of the blend are performed successively. This principle is, however, not applicable to most of the conventional molding machines for general purpose in which the step of extrusion or injection is not always preceded by the step of thorough kneading of the blend.
For example, an attempt to introduce a blend of a filler and single-component pellets of the resin alone is unsuccessful due to the large segregation of the components as mentioned before while an attempt to introduce a blend of a finely powdered resin and a filler is also impractical, although the distribution of the filler is somewhat improved in comparison with the blend of resin pellets and a filler, problems arise because of the extreme retardation of the feeding rate to the feeder screw of the molding machine resulting in impractically lowered effeciency of production.
Further, there has also been a problem in the process of mixing or kneading of a thermoplastic resin and a filler in a mixing machine such as a Banbury mixer as a preceding step of the production of the resin composition into shaped articles. In carrying out the mixing in such a mixing machine, weighed amounts of the components are introduced into the mixing chamber of the machine and the chamber is closed by lowering a cover called "float" followed by the rotation of the rotor and heating by means of the jacket to effect plasticization or gelation of the resin into which the filler is incorporated by the shearing force given by the rotor. Since fillers are usually in finely divided powdery form, it is unavoidable that a considerable amount of the filler is scattered during descending of the float or during the period before the gelation of the resin through the gaps between the float and the jacket leading to the loss of materials. To avoid this difficulty, it is sometimes practiced to move the float up and down several times before the beginning of gelation so that the filler scattered on the float can be brought back into the chamber. This method is, however, very troublesome for the relatively small effect in addition to the disadvantages of rather accelerated scattering of the filler by the movement of the float and intrusion of foreign matters accompanying the movement of the float.